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These images show the dramatic decline of Lake Chad, Africa, which lies on the borders of four countries: Nigeria, Niger, Chad, and Cameroon. Lake Chad was once the sixth largest lake in the world, but prolonged drought and increased water use have shrunk the lake from about 25,000 km2 in 1963 to less than 2,000 km2 today.

In the Landsat images, open water has been replaced by wetlands, which are shown in green. The desert appears tan, and the shallow lake water is blue. The black and white 1963 image is from the film-based Argon reconnaissance satellite, declassified in the 1990s.

Lake Chad’s lakebed is flat and shallow, so small changes in water level greatly affect its area. Even in normal times, Lake Chad was no more than 5–8 m deep. It may be more accurate to think of it as a deep wetland. Considered this way, Lake Chad was once the second largest wetland in Africa, highly productive, and supporting a diversity of wildlife.

Lake Chad has a large drainage basin (2.5 million km2), but almost no water flows in from the dry north. The Komadougou-Yobe River, northwest of the lake, now flows only in the rainy season. About 90% of Lake Chad’s water comes from the Chari River, which flows into the lake from the southeast. The Chari averaged about 40 billion m3 per year from the 1930s to the 1960s but now averages only about half that.

Even though it has no outlet to the ocean, Lake Chad is a freshwater lake. The Chari River puts few dissolved solids into the lake, as many of its suspended solids drop out onto its wide floodplain. Once in the lake, some dissolved solids precipitate, and some are absorbed by plants. Also, 5–10% of the lake water seeps away through the ground, carrying away dissolved solids with it.

Low-rainfall regions are usually also variable-rainfall regions. When rains fail, as in 1972, Lake Chad water levels drop rapidly because annual inflow is 20–85% of the lake’s volume. Much of the soil in the Chari basin is clay particles that swell together when wet, so water runs off rapidly rather than percolating slowly. There is naturally some delay between upstream rainfall and the resulting rise in lake level. About 90% of the rain falls from June to September, but the lake suddenly rises in November. Highest lake levels are in December, tapering off slowly for several months.

Fluctuations are not new to Lake Chad. About 10,000 years ago, Lake Chad almost filled its present drainage basin, and spilled southwest out the Benue River to the Atlantic. In the last 1,000 years, according to fossil evidence, the lake probably dried out a half-dozen times. (Most of its fish are river-adapted species.) Geologic data, climate data, historical accounts, and reconstructions all indicate a higher long-term variability than the relatively short period we have actually measured. The chart shown here shows levels since the 1870s, from actual measurements and from estimates based on Nile River discharge.

Following highs in the 1870s and 1890s, the lake dropped enough by 1908 to separate into north and south pools, with the “Great Barrier” between. In the 1950s, the lake rose enough to flood irrigation systems, peaking this century in 1962. The lake then tapered off until the early 1970s, when it plummeted. The recent low levels are a concern and have been monitored through satellite and other means by the Lake Chad Basin Commission and others.

People around Lake Chad are among Africa’s most chronically vulnerable to food insecurity. They deal with variability through mobility and through diversity of food sources. Lake-related activities include fishing and soda mining. Some people raise livestock, typically moving closer to the lake for grass in the dry season, then moving away in the rainy, mosquito season; some will graze their animals up to 100 km away. After the 1970s droughts, herders shifted from grazing animals (cattle and camels) to browsing animals (sheep and goats), which affected the area’s vegetation by consuming the woody plants.

Crop strategies include farming the lake bottom; the receding lake left behind an estimated .5 million ha of cultivable land, some of which was being cropped. Farming is also done on “recessional lands,” where the lake water recedes every year, in the “polder” depressions between dunes. Rice, wheat, maize, and vegetables are grown. In a traditional polder, one crop a year is grown as the lake water recedes. If dams and pumps are used, up to three crops a year can be grown. Besides fewer fish, a low lake also means a shorter shoreline and thus fewer polders. Chad’s Lac Prefacture estimated that only 10% of its polder areas were being used.